The present invention relates generally to methods and apparatus that may be used for evaluating a plasma, and more specifically relates to such methods and apparatus providing improved evaluating of plasmas, particularly those for which previous measurement techniques have offered less than optimal results.
A number of systems have been used or considered for evaluating of plasmas formed in chambers or similar devices. In many circumstances the systems have been configured to measure relatively high temperature and/or high density plasmas. For example, systems such as Heavy Ion Beam Probes have been used for such purposes. While different configurations of Heavy Ion Beam Probe systems are known for evaluating different types of plasma devices, in general, such systems operate by directing an ionized beam through the plasma, where the ions will become “heavy ions” through electron impact within the plasma, thereby becoming “doubly ionized”. The doubly ionized particles will then be deflected by the magnetized plasma, and detected by an energy analyzer which can then discern the energy gained by the ions, and from that energy data identify the electric potential of the plasma at the point of ionization.
One limitation of such Heavy Ion Beam Probe systems is that the incident electron impact energy must be equal to or greater than the second stage ionization potential of the ions in the beam. Thus, a significant limitation on such Heavy Ion Beam Probe systems is that they are not suitable for use with relatively low temperature plasmas, for example, plasmas operating at approximately 1-2 eV. Such plasmas typically do not have enough sufficiently hot electrons to achieve significant second stage ionization in the ion beam, and thus signal levels are generally very low.
This limitation on such probe systems is significant particularly to industries such as the semiconductor manufacturing industry, which typically uses “cold plasmas,” that is plasmas with electron temperatures of approximately 2 electron volts or less. However, measurement of plasma characteristics is very important in the semiconductor industry because the plasma may have a significant impact on the semiconductor manufacturing process. Because of this need to measure these cold plasmas, the most common techniques currently used to measure the plasma in semiconductor systems have used Langmuir probes inserted into the plasma. With such probes, however, the measurements are less than optimal because the mere presence of the Langmuir probe disrupts the plasma to at least some degree. Additionally, the measurement may only be made at a single point in the plasma, and is often believed to be inaccurate.
An additional concern arises in some applications, and is exemplified in the semiconductor manufacturing industry, where the real need is to evaluate the plasma potential (voltage) and density across a geometrical dimension. For example, in semiconductor manufacturing, the vast majority of such manufacturing is done by depositing or otherwise forming a succession of patterned layers on a circular substrate such as a thin silicon wafer. Many forms of deposition operations and patterning operations, such as etching, involve the use of plasma mechanisms. The current conventional technology forms such layers on a 300 mm diameter wafer, and a critical factor in such manufacturing is the consistency of deposition or etching operations across the entire dimension of that wafer. Additionally, a varying plasma gradient across the wafer may create localized plasma charging resulting in damage to the structures formed, such as the gate oxide layer. Accordingly, for that industry, the currently-available techniques do not provide either a system capable of measuring the relatively cold plasmas that are typically of interest, or a mechanism for identifying any irregularities or discontinuities in plasma characteristics across the wafer dimension. While similar needs are believed to be experienced in other industries, the semiconductor manufacturing industry provides an accessible and understandable example of where currently-known plasma measurement methods and systems fail to provide capabilities that would be beneficial to the industry.
Accordingly, the present invention provides new methods and apparatus for evaluating plasmas which is capable of evaluating relatively cold plasmas, as well as hotter plasmas; and which in some examples provide additional capabilities of profiling plasmas across the dimension of the plasma.